Thanks, Imber - this is definitely of interest. From the dates I think this must be John Three, John Two's son born 1868, since John his father left Dublin in 1896 for South Africa.
When this was first posted by ShaneW (on 11 May) I wasn't far enough on to be able to connect it at all with anything I had at the time. I've since found that my uncle's notes were not all accurate and in this case I think he has confused two Johns again.
John Two the father was probably selling bottles to publicans who then filled them with Guinness, from what I find online.
John Three could well have been bottling for Guinness and have been connected with the Hibernian Glass Bottle Works, which is now very helpful.
I have done a search for the Hibernian GBW in a Dublin newspaper available via the library and I found one mention in 1883, but nothing else until 1896 and later. (Possible that it was being planned then, 1883, but I would have to check back.)
I haven't found any connection of John Two to this works, but am still trying to find out if he was at the Ringsend Bottle Works or the Irish Glass Bottle works - connections to both, all works at Ringsend.
I now don't think he remarried (25 May), I think the difficulty was that he joined the Plymouth Brethren, and after that there are no more christening records. I'm now satisfied that the three sisters I was looking for were my grandfather's full sisters.
To go back to the last post, I will investigate E & J Burke and see if there are any helpful dates there re the works.
Thanks again - Alison